1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile system or a composite OA (office automation) machine, and in particular, to an image forming apparatus adapted to aim at reducing noise during operation.
2. Description of the Related Art
An image forming apparatus such as a copying machine or a printer includes several movable components such as motors, gears, rollers and the like, which are incorporated in a paper feed portion, an image forming portion, a fixing portion and the like. These components cause vibration which is a source of noise. Further, this vibration also vibrates other components, resulting in more noise within the image forming apparatus.
For example, a typical image forming apparatus may have a plurality of optical scanning units each having a polygon mirror motor which creates noise. Image writing positions of light beams coming from surfaces of polygon mirrors in the plurality of scanning units of the image forming apparatus have to be aligned to one another, and accordingly, the writing positions of light beams coming from surfaces of the polygon mirrors are detected. Further, in a certain image forming apparatus, polygon mirror motors for rotating the polygon mirrors are provided thereto with speed detecting elements, respectively, in order to rotate the polygon mirrors at an equal speed (Refer to Japanese Patent Laid-Open No. 09-230273).
Next, an explanation will be made herein below with regard to a conventional polygon mirror motor assembly 200 which is typically part of an image forming apparatus having a plurality of optical scanning units with reference to FIG. 2. The polygon mirror motor assembly 200 includes a rotating polygon mirror 101, a motor 102, a rotary shaft 103, a leaf spring 104 for pressing and fastening the rotating polygon mirror 101 to the rotary shaft 103, and a circuit board 105 of which the motor 102 is mounted thereon.
A predetermined radial gap (or tolerance) is disposed between the outer diameter of the rotary shaft 103 and the inner diameter of the rotating polygon mirror 101 in view of working efficiency during assembly on a mass production basis of the rotating polygon mirror 101 onto the motor 102 and the rotary shaft 103. Further, similarly, a predetermined radial gap is defined between the leaf spring 104 and the rotary shaft 103.
However, the aforementioned gaps are disadvantageous because they cause the polygon mirror motor assembly 200 to have an imbalance problem. That is, the rotary polygon mirror 101 fitted on the rotary shaft 103 is movable more or less in a radial direction due to these gaps, resulting in weight imbalance.
Conventionally, it has been known that the rotating polygon mirror 101 may be coated on its top surface with an imbalance correction putty (ultraviolet curable resin) as one of measures for improving the weight imbalance of the polygon mirror motor assembly 200. However, in view of the weight of the imbalance correction putty, the degree of accuracy of the coating, and the like, the aforementioned remedial attempt to achieve an adequate balance is still typically insufficient to improve the weight imbalance. Further, in order to ensure a nominal yield, it is necessary to tolerate or accept polygon mirrors 101 which have an inherent imbalance problem. That is to say, conventional polygon mirrors 101 are typically produced and shipped with an inherent imbalance problem more or less.
Therefore, a typical image forming apparatus having a plurality of optical scanning units, each including a polygon mirror having the above-mentioned configuration, has several problems associated with it. For instance, when the rotating polygon mirrors 101 are rotated, the plurality of optical scanning units vibrates, due to weight imbalances which are respectively inherent to the polygon mirrors 101 of the optical scanning units. It is noted here that since the polygon mirrors 101 are adapted to be subjected to PLL (Phase-Locked Loop) control, the polygon mirrors 101 are rotated with rotated phase relationships of the polygon mirrors 101 being fixed so as to cause the surface phases of the rotating polygon mirrors 101 to be equal to each other. Accordingly, the phase relationship in the direction of the weight imbalance causes arbitrary phase difference, and is changed each time when the image forming apparatus is energized.
Another problem pertains to the vibration from the optical scanning units, caused by weight imbalance of the polygon mirror motors 102 which is transmitted to a housing serving as a structure for supporting the rear surfaces of the optical scanning units, resulting in generation of noise. It is noted, as stated above, that the phase relationships of the polygon mirror motors 102 in their weight imbalance directions are arbitrary, and accordingly, the phase relationships are changed each time when the image forming apparatus is energized. As a result, the noise is also changed each time when the image forming apparatus is energized.
In an effort aimed at reducing noise, there has been proposed an image forming apparatus in which acoustic members are provided in a housing, and as well acoustic members disposed on an inner bottom surface and side surfaces of a paper feed tray in order to absorb sound generated in the paper feed tray during paper feed. Thereby it is possible to reduce noise (refer to, for example, Japanese Patent Laid-Open No. 2003-89467).
However, in the above-mentioned conventional example, acoustic members, sound suppressing members and the like are added in the apparatus, and as a result, the volume of the apparatus is increased, and the amount of work during assembly of the image forming apparatus is also increased so as to increase the costs of production.
In light of the aforementioned drawbacks of the conventional approaches of reducing unwanted noise, it would be beneficial to provide an image forming apparatus which is inexpensive and space-saving and which may exhibit noticeable noise reduction effects, with no provision of vibration preventing members, acoustic members, sound suppressing members and the like.